It is proposed to continue studies on the biosynthesis of the ubiquinones in rat liver mitochondria, rat brain and in bacterial systems. Special emphasis will be directed to the reactions involving polyprenylpyrophosphates as precursors of the isoprenoid side chain of the ubiquinones. The first step in ubiquinone-9 biosynthesis is catalyzed by the enzyme, p-hydroxybenzoate:polyprenyltransferase in which the 3 position of p-hydroxybenzoic acid is alkylated by solanesyl pyrophosphate to form 3-nonaprenyl-4-hydroxybenzoic acid. It is planned to purify and study this enzyme which has been found in the inner membrane of mitochondria. Utilizing agents which combine with polyprenylpyrophosphates such as bacitracin, further studies will be made on the significance of the presence of polyprenylpyrophosphates in inner mitochondrial membranes and the possible role that they might play in a variety of processes occurring in mitochondria. Ubiquinone biosynthesis has been shown to take place in rat brain slices and the same reactions which occur in rat liver mitochondria have been detected in brain mitochondria. These will also be studied in the same manner. Various analogues of p-hydroxybenzoate have also been found to inhibit the alkylation reaction. The mechanism of this inhibition will also be studied.